This application relates to and incorporates herein by reference Japanese Patent Applications No. 10-217607 filed on Jul. 31, 1998, No. 10-290392 filed on Oct. 13, 1998 and No. 10-356324 filed on Dec. 15, 1998.
1. Field of the Invention
The invention relates to a fuel injection system for directly injecting fuel into a cylinder of an internal combustion engine and, more particularly, to a fuel injection system for performing fuel injections a plurality of times per cycle of an internal combustion engine.
2. Related Art
There is known a fuel injection system which accumulates high pressure fuel supplied from a fuel pump in a common rail, and supplies the accumulated high pressure fuel to a fuel injector which responsively open at a predetermined fuel injection timing, thereby directly injecting the fuel into a cylinder of an internal combustion engine.
This fuel injection system is used not only for a diesel engine for compressing and igniting fuel mixture but also a gasoline engine for igniting the fuel mixture by a spark generated by a spark plug. In recent years, combustion efficiency is improved by executing an injection of fuel from the fuel injector by a pre-injection and a main injection per cycle of the internal combustion engine, as proposed in JP-A 4-252830 and SAE 980505, for instance.
In such conventional fuel injection systems, however, since it is set so that the half or more of the entire fuel to be injected into the cylinder of the internal combustion engine is injected by the pre-injection and the fuel is distributed into the whole area in the cylinder, the mixture in the whole area in the cylinder becomes lean by the pre-injection. The fuel mixture is partially pre-ignited by compression in the cylinder by movement of a piston, thereby tending to cause knocking. When the occurrence of the knocking is tried to be suppressed, preferable engine performance cannot be obtained.
In another fuel supply system according to U.S. Pat. No. 5,694,903 for supplying a fuel from a high pressure fuel supplying pump to a fuel injector, a needle lift is changed by a fuel pressure to vary a characteristic of the rate of injection. In this fuel injection system, however, it is necessary to change a driving power supplied to a solenoid in two phases, thus resulting in a complicated and expensive driving system.
Although the rate of injection can be variably controlled, variable control of spray conditions such as the angle of spray and spray penetration cannot be realized. When the spray conditions are not properly controlled, the fuel consumption worsens and production of NOx, soot, HC, and the like increases.
Further, JP-U-5-24956 discloses a swirl type fuel injector for promoting atomization of fuel and controlling spray characteristics, for example, a fuel injector in which a member for swirling a fuel is provided in the nozzle body, JP-A 3-175148 discloses a fuel injector in which a needle is provided with a fuel swirling mechanism, and U.S. Pat. No. 5,170,945 discloses a fuel injector in which a spray pattern of a fuel is changed by a lift of the needle. It is, however, difficult to obtain a preferable spray pattern.
It is a first object of the invention to reduce NOx, HC, and soot exhausted from an internal combustion engine and, further, to improve fuel consumption and output of the internal combustion engine while suppressing occurrence of knocking due to pre-ignition of fuel mixture injected by a pre-injection.
It is a second object of the invention to provide a fuel injection system which can supply fuel at a proper rate of injection under proper spray conditions in accordance with the operating conditions of an internal combustion engine.
It is a third object of the invention to provide a fuel injection system using a fuel injector whose angle of fuel spray is variable.
In a fuel injection system according to a first aspect of the invention, when a pre-injection and a main injection are executed, a fuel injector is controlled so that fuel injected by the pre-injection is formed into a stratified spray of uniform mixture concentration in a predetermined zone in the cylinder at an ignition timing after the pre-injection, and fuel injected by the main injection is ignited in a combustion zone of the stratified spray made by the pre-injection, passes through the combustion zone, and reaches a combustion chamber in a piston, which has therein un-used air.
In order to stratify the fuel spray injected by the pre-injection, for example, it is sufficient to widen the angle of fuel spray from the fuel injector and weaken the penetration of the fuel spray so as not to diffuse the fuel spray in a wide range after the fuel injection. In order to allow the fuel spray injected by the main injection to penetrate the combustion zone of the stratified spray made by the pre-ignition, it is sufficient to narrow the angle of fuel spray from the fuel injector to thereby enhance the penetration of the fuel spray.
In a fuel injection system according to a second aspect of the invention, a needle which opens and closes the nozzle hole is lifted in stages, thereby changing the rate of injection and the spray conditions of a fuel injected according to the lift of the needle. The spray characteristics such as the rate of injection, the angle of spray, and the penetration can be changed according to the needle lift amount and a fuel can be atomized at low pressure. Thus, the fuel consumption can be improved and occurrence of NOx, soot, HC, and the like can be reduced.
In a fuel injection system according to a third aspect of the invention, a first swirl generating chamber is formed in a nozzle body, and a first swirl generating part having an outer wall in which a first fuel inlet passage communicated with the first swirl generating chamber is formed is provided on the outer surface of a needle. The outer surface of the first swirl generating part in which the first fuel inlet passage is formed faces the inner surface of the nozzle body in which a valve seat member is formed. Consequently, according to a lift of the needle, the distance between the outer surface of the first swirl generating part and the inner surface of the nozzle body in which the valve seat member is formed changes, and the opening area defined by a contact part of the needle and the valve seat member changes. An axial component of fuel flowing through the first swirl generating part is thereby adjusted and the ratio between the circumferential component of fuel flowing through the first fuel inlet passage and an axial component of fuel flowing through the first swirl generating part is adjusted.